Although not limited to a specific area of semiconductor technology, the present invention is of particular relevance in the manufacture of structures of the sSOI type (strained silicon on insulator) using the Smart Cut® technology to obtain the desired hetero structure. Examples of the implementation of Smart Cut® technology are described in, for example, U.S. Pat. No. 5,374,564 or A. J. Auberton-Hervé et al. intitulé “Why can Smart-Cut Change the Future of Microelectronics?”, Int. Journal of High Speed Electronics and Systems, Vol. 10, No. 1, 2000, p. 131-146.
This technology involves the following steps:
a) bombardment of one face of a donor substrate (for example, silicon) with light ions such as those of hydrogen or helium in order to implant these ions in sufficient amounts in the substrate, the implant zone generating a zone of weakness through formation of microcavities or platelets,
b) bonding of this face of donor substrate with a receiver substrate, and
c) cleavage/fracture of the donor substrate about the implanted zone enabling the transfer to the receiver substrate of that part of the substrate situated between the surface subjected to ion implantation and the zone of weakness generated at a certain depth via the implantation. A silicon-on-insulator (SOI) structure can thus be obtained.
In a typical application of Smart Cut® technology, this may involve first of all the preparation of a donor substrate comprising a support substrate of silicon upon which a buffer layer of SiGe (silicon-germanium) is prepared, and on top of this a relaxed layer of SiGe. The buffer layer has a very low level of germanium at the interface with the support substrate and, at the other interface, a level of germanium close or equivalent to that of the germanium in the relaxed layer.
Upon the relaxed SiGe layer, epitaxy can be used to form a layer of strained silicon. Strained silicon, where silicon is grown on a Si—Ge surface, and is obliged to have more broadly spaced atoms than in pure silicon, shows higher charge carrier mobility and therefore increased transistor operation speed.
The assembly of layers comprising the three mentioned (sSI/SiGe/Si) is used as a donor substrate in the Smart Cut® technology. This is then bonded to a suitable acceptor substrate, and the original silicon and SiGe layers of the donor substrate are removed in the preparation of the final wafer, comprising strained silicon on a receiver substrate.
In this system, the concentration of germanium in the relaxed layer may vary according to the degree of strain required in the strained silicon layer.
Before the growth of the strained silicon layer by epitaxy, cleaning of the SiGe surface by the HF-last procedure is commonly used. The HF-last procedure removes oxide and makes the surface hydrophobic. A polishing of the relaxed SiGe layer before application of the HF-last is commonly carried out. The strained silicon layer is in general rather thin, having a thickness of the order of 200 Å. It is therefore important to be able to control the quality of this layer as much as possible. After removal of the SiGe relaxed layer during preparation of a final sSOI substrate, the contact surface between the strained silicon grown by epitaxy and the original SiGe will once again be exposed, and if there are defects, these will be exposed in the final sSOI product.
It may also be noted that the sSOI surface here may be the subject of further epitaxy (“re-epitaxy”) after transfer onto the receiver substrate, but this may not suffice to annul the consequences of the original surface defects.
It is in this context that the HF-last treatment of the SiGe layer before epitaxy is important. It is known that this procedure may give rise to surface defects known as “watermarks”. Various causes are believed to be behind the appearance of watermarks, such as the presence of oxygen dissolved in the deionised water used for rinsing, or hydrofluoric acid vapor present in the vicinity of the wafer during drying thereof. These factors are discussed in the article by Namba et al., “Insights Into Watermark Formation And Control”, Solid State Phenomena, Vols. 103-104 (2005) pp. 83-86.
It is known at present to carry out, after the HF-last treatment, a thermal treatment at high temperature (about 800° C.) using an H2 bake to remove remaining oxygen. This treatment, requiring a certain amount of energy consumption, does not however completely solve the problem of watermarks.
US 2007/0256705 discloses wet cleaning of a semiconductor surface involving successive treatments with HF solutions and then solutions containing strong oxidizing agents. The use of hydrochloric acid (HCl) in the presence of HF is taught to reduce roughening of the surface, or the tendency to form pitting, due to oxidation of a silicon surface in the presence of noble metals. Exemplified relative concentrations are 0.2% HF and 1.0% HCl. The use of megasonic waves during the phases of substrate treatment involving oxidizing treatments is disclosed by US 2007/0256705, for example during treatment with an ozone-containing solution.
U.S. Pat. No. 5,932,022 relates to a multi-step process for producing a silicon wafer with a hydrophilic oxide surface layer. The multi-step process, apart from rinsing and drying layers, involves a first application of an oxidizing solution (NH4O: H2O2: H2O), followed by the application of an aqueous solution containing hydrofluoric acid (HF) and hydrochloric acid (HCl), followed by a final application of an oxidizing solution (containing both H2O2 and HCl). The treatment with HF and HCl thus produces an intermediate hydrophobic surface, prior to formation of the final hydrophilic oxide-bearing surface. U.S. Pat. No. 5,932,022 teaches that HCl enables removal of metals from the surface through the formation of soluble metal complexes. The use of megasonic waves during the treatment of the surface with the first oxidizing solution (NH4O: H2O2: H2O) is exemplified in U.S. Pat. No. 5,932,022.
U.S. Pat. No. 5,051,134 discloses the use of cyclodextrins to reduce particle contamination of semiconductor surfaces such as silicon wafers during treatment by hydrofluoric acid-containing aqueous solutions.
A number of prior art cleaning processes for cleaning semiconductor surfaces are thus known, but the above-mentioned references either do not address the specific issue of watermarks, are not related to providing hydrophobic surfaces as final products, in particular for further epitaxial growth, or require further mandatory components in the cleaning compositions to achieve a technical effect. Megasonic waves have been disclosed in some of the above-mentioned in some cleaning processes, but not in relation to watermarks nor in the framework of HF-based treatment stages giving rise to hydrophobic surfaces, as opposed to oxidizing treatment stages giving rise to hydrophilic surfaces.